In a conventional chemical vapour infiltration (“CVI”) process for aircraft brake manufacturing, a large number of porous substrates (frequently referred to in the art as “preforms”) are placed in a graphite reaction chamber heated by an inductive or resistive heating source to a temperature of about 900° C. to about 1000° C. A precursor gas containing one or more carbon precursors (typically hydrocarbon gases such as methane and/or propane) is admitted into the graphite reaction chamber. The precursor gas or gases are preferably preheated before entering the reaction chamber to a temperature range between about 500° C. and 950° C., and in a particular case, between about 500° C. and 750° C., by a gas preheater in order to minimize a thermal heat loss from the precursor gas. An example of an appropriate gas preheater in this regard is described in U.S. Pat. No. 6,953,605.
In a conventional CVI process, the substrates may require as many as several weeks of continual infiltration processing. One or more intermediate machining steps may also be required to reopen the porosity of the substrates by removing the “seal-coating” that prematurely seals the surface of the substrates and prevents further infiltration of the reactant gas into its inner regions. Important process variables in a CVI process includes substrate temperature and porosity; the flow rate, temperature, and pressure of the precursor gas(es); and reaction time. A particularly important parameter is the substrate temperature. A common problem in CVI densification is that the preforms are not uniformly internally densified. This frequently occurs when the preform substrate temperature has a large gradient.
In addition, the efficiency of conventional gas preheaters may not be as good as desired.
An example of a conventional densifying process relates to densifying undensified substrates, such as annular preforms, and/or partially densified substrates (including annular preforms). The undensified substrates are sometimes referred to in terms of undergoing a first infiltration step or an “I-1” step for short. Likewise, the partially densified substrates undergo a second infiltration step, or an “I-2” step. The annular substrates are arranged in several stacks in the reaction chamber, for example, above a conventional gas preheater.
Examples of conventional loading are illustrated in FIGS. 1 and 2, in which a given tray in the furnace is stacked with either all I-1 or all I-2 substrates. FIGS. 3 and 4 are histograms corresponding to FIGS. 1 and 2, respectively, illustrating the number of substrates on the tray attaining a given density. Several trays, each having several stacks of porous substrates arranged thereon, are in turn stackingly arranged in the furnace. For example, seven trays may be provided.
Full I-1 and Full I-2 Loading Configuration (Conventional Art):
In the arrangement of FIGS. 1 and 2, about 1100 porous substrates, ±100, may be provided in total in the furnace. Densification time may be between about 475 hours and 525 hours. Only I-1 or only I-2 parts are treated at one time. After initial I-1 densification, a separate milling step is required to “reopen” the porosity of the substrates after the I-1 densification step.
A large thermal gradient occurs in the horizontal, or transverse, plane because of poor thermal conductivity and low thermal mass of the fiber preforms. The substrates on the bottom and top trays are relatively poorly densified, while lateral stacks on intermediate trays 2-6 are best densified. Between 30% and 40% of I-1 parts have a bulk density ranges between 1.30 g/cc to 1.40 g/cc. See FIG. 2 for example. Fiber pull-out or delamination around the ID and OD of the pre-densified disk is commonly seen from the intermediate machining operation mainly because I-1 density is too low.
However, temperature gradients may be observed in the reaction chamber in both vertical and horizontal planes, such that the temperature close to substrates in the central stacks (in a radial or horizontal sense) may be at least several tens of degrees C. lower than the temperature of the lateral (i.e., radially exterior) stacks. For example, stacks located in the central part (in a horizontal sense) of the reaction chamber may not benefit from the heat radiated by the susceptor as much as the stacks located closer to the internal side wall of the susceptor. This can cause a large temperature gradient, and consequently, a large densification gradient between the substrates stacked on the same loading plate. FIGS. 5 and 6 illustrate examples of the temperature gradients conventionally present in the horizontal and vertical directions, respectively.
In order to solve this problem using conventional approaches, the size of the gas preheater could be increased to further improve the heating of the substrates. However, if the gas preheater is an internal device (relative to the reaction chamber) this approach reduces the useful load capacity in the furnace, which in turn reduces the number of substrates being treated.
Another problem is the formation of undesirable carbon microstructures, such as smooth laminar carbon, soot, and tar. These type microstructures are not desirable because of their poor thermomechanical and friction properties. These kinds of problems may be attributed to long precursor gas residence times, and to temperature variations in the deposition environment.
Finally, gas preheating can actually create undesirable effects if the temperature of the precursor gases is raised close to the reaction (i.e., deposition) temperature. In particular, the precursor gas or gases may prematurely break down and deposit carbon soot and the like on the surface of the processing equipment or even on the exterior of the preforms. All of these results negatively affect the efficiency of the process and the quality of the resultant articles.